1. Field of the Invention
This invention relates to a device and method for facilitating measurements of the solubility of organic, inorganic and organic-metallic compounds, and for performing titration studies on small amounts of samples of such compounds, particularly of compounds related to pharmaceutical research and development.
2. Description of the Prior Art
In the field of pharmaceutical research and development, it is almost always necessary to evaluate the general suitability of a newly developed drug candidate prior to launching into full development. Evaluation of the general suitability of such chemical compounds typically includes solubility studies of the compound in various in-vivo compatible or physiologically compatible solvents, as well as other chemical characteristics, such as the determination of the acid dissociation constant (pK.sub.a) and solubility profiles at various pH values. Due to the lack of knowledge regarding the fundamental chemical properties of new drug candidates, the cost of purifying or manufacturing such drug candidates is generally extremely high, and therefore the quantities of compound produced in initial research stages are very limited.
The current process for determining the solubility of a given compound in a solvent such as water, for example, is as follows: A 1.5 to 2.0 milliliter (ml) glass vial is selected, into which is placed a known amount of distilled water, typically about 0.5 to 1.0 ml. Then, an amount of the compound is added to the vial, the amount being substantially greater than the amount estimated to be the compound's solubility in water. For example, if it is estimated that a compound's solubility in water is 10 milligrams (mg) per ml, the amount of compound to be added to the vial should be at least 15 to 20 mg. Using substantially more compound than is estimated to be necessary to saturate the solvent insures that when the compound-solvent system has reached equilibrium, there will remain in the vial at least some amount of undissolved compound. Following the introduction of the compound into the vial, the vial is capped and the vial is placed in a rotor/shaker. The temperature is held at a constant temperature, commonly either 25 or 37 degrees Centigrade, and the vial is shaken or rotated for a certain period of time, typically at least 24 hours.
After the certain period of time, the sample is inspected to verify that solid compound remains in the vial. If no solid compound remains, the process must be reinitiated with additional compound and the vial must again be shaken for a certain period of time. Assuming that solid compound remains, the solid compound and solution are separated by either filtration or centrifugation.
Filtration is usually performed by hand using a standard syringe and syringe-adaptable filter. Regarding the filter used, it is generally necessary to predetermine the degree of adsorption of the compound on the filter during filtration. If the compound in question has a density different from the solution, centrifugation may be employed. Preliminary analysis is necessary in order to determine the necessary centrifugal force to ensure complete separation. After separation by either filtration or centrifugation, the concentration of the compound in the solution is determined by a suitable analytical method to arrive at its solubility.
There are numerous drawbacks with current methods (and related devices) for evaluating the solubility of a compound in a particular solvent. The primary drawbacks are threefold. First, the current methods and devices require relatively large quantities of solvent which in turn means that relatively large amounts of compound are necessary. Second, the current methods and devices require a time-consuming and wasteful process of separation of the saturated solution from the undissolved compound. More specifically, if filtration is used, compound adsorption on the separation filter must be considered. If centrifugation is used, centrifugation speed and material densities must be determined, and the possibility that fine particles of solid remain in the supernatent must be considered.
The third and perhaps most significant drawback with the present methods and devices is the repetition of the above-described process. The entire process described above is normally repeated for various periods of time to insure an accurate and complete solubility measurement. For example, FIG. 1 shows a typical solubility curve 101 over a period of time for a typical chemical compound in a solvent such as water. With respect to FIG. 1, it is readily appreciated that to accurately measure the solubility of the chemical compound, it is very important to allow the chemical compound to mix with the solvent for a sufficiently long period of time in order to reach an equilibrium solubility 102. However, because the chemical compound is "new" and many of its chemical properties are unknown, the amount of time necessary to attain such saturation of the solvent is uncertain. Furthermore, as seen in FIG. 2, not all solubility curves 111 simply approach the equilibrium saturation plateau 112. The solubility curve 111 in FIG. 2 has a local maximum (or overshoot) solubility at point 113, however, point 113 is not the equilibrium solubility of the compound in the solvent.
Thus, repetition of the solubility analysis further consumes additional quantities of compound. To further illustrate the relatively large quantities of chemical compound required in the aforesaid normal or standard practice, if five different solvents are intended to be evaluated for their solubility properties with respect to a particular compound, the average solubility of the compound in each solvent is estimated to be 10 mg/ml, and the quantity of solvent to be used is 0.5 ml, at least 35 to 50 mg of the compound will be required. Furthermore, to assure accuracy (see, e.g., FIGS. 1 and 2), the samples should be tested in triplicate, which will consume 105 to 150 mg of compound for a solubility study in only five different solvents. Moreover, if the compound needs a long period of time to reach equilibrium, an additional amount of compound would be required to repeat the testing. As explained above, however, the available quantities of newly developed compounds are very limited. In the preparation of new compounds for pharmaceutical research, it is not uncommon to be limited to 200 mg or less for solubility and characterization studies, which may not even be enough to generate the basic solubility data.
Therefore, it is very important to consume as little compound as possible while repetitively measuring the solubility of the compound in the solvent over different periods of time to insure that the equilibrium saturation point has been reached. It is also important to repetitively measure the solubility of the compound to insure accuracy of the prior measurements. These repetitive measurements, each using relatively large amounts of chemical compound, demonstrate a major shortcoming of the aforementioned current practice of measuring solubility.
In view of cost, availability and accuracy factors, it would be desirable to use small amounts of the subject chemical compound and correspondingly small amounts of solvent for studies involving solubility. It would be desirable to eliminate the separation step before determining the concentration of the compound in the solution. It would also be desirable to generate a solubility curve (again see, e.g., FIGS. 1 or 2) during the solubility study while keeping the necessary amount of compound to a minimum.
Titration, another common analysis performed on solutions of new chemical compounds, is generally performed by adding a given amount of acid or base solution into the solution. After thoroughly mixing the solution and assuming it reaches equilibrium, the pH value of the mixed solution is determined using a pH-electrode. These steps are repeated until the final solution reaches a desired pH. Due to the requirement of thorough mixing, the volume of solution in a titration study is normally in the range of milliliters. It would be desirable to reduce the volume of solution required for a titration study.
Another type of titration study, commonly called "suspension titration", involves evaluating the solubility of solid compounds suspended in a solvent, at various pH values of the solvent. Suspension titration is a combination of the previously described solubility and pH titration procedures. Following the addition of an acid or base titrant to the suspension of a given acidic or basic compound, the pH is monitored until a constant value is obtained (indicating that equilibrium has been established). The concentration of the dissolved solute is then determined as described above. Another aliquot of acid or base is then added and pH is again monitored until a constant value is again attained. Solute concentration is then again determined at this new pH. By repeating this procedure until the desired pH range is fully covered, a pH solubility profile can be derived using the same solid sample of the compound for all points. A typical pH solubility profile is illustrated in FIG. 5.
Devices for performing titration, sampling and solubility measurements on small quantities of materials have been described in the prior art. For example, U.S. Pat. No. 2,946,486 to Gilmont describes an analytical device for titrating functions comprising a micro-buret having capillary tubing at one end and an enlarged chamber at the other end.
U.S. Pat. No. 4,086,062 to Hach shows a liquid dispensing device for chemical titration having a reciprocating plunger for expelling fluid from an attached titrating solution cartridge.
U.S. Pat. No. 4,743,570 to Machler et al. shows a flow-through cuvette having a very small volume for use in high-pressure liquid chromatography.
Further, Smith et al. in U.S. Pat. No. 5,045,284 disclose a flow injection analysis flow cell for titration flow injection analysis.
Upon consideration of the aforementioned disclosures, it will be observed that none of the described inventions and patents, taken either singly or in combination, may be regarded to describe or to suggest the instant invention as claimed.
Accordingly, it is a principal object of the invention to provide a device and a method for facilitating measurements of the solubility of chemical compounds in various solvents, wherein such compound samples are present in small amounts.
It is another object of the invention to provide a device and method for determining various characteristics of solutions, particularly of compounds in solution, and in particular, for example, the pH of solutions wherein such solutions are present in submilliliter amounts.
It is another object of the invention to provide a device and method for determining various characteristics of solutions without an additional or separate procedure for separating the saturated solution from the undissolved chemical compound.
It is yet another object of the invention to provide improved elements and arrangements thereof in an apparatus for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.